Save the Spring is a partnership between the River Dee Trust, Dee District Salmon Fishery Board and Atlantic Salmon Trust, supported by the University of Stirling and UHI Inverness.
A recap and update on the programme’s smolt-to-adult supplementation trial. The story so far, lessons learned and what’s in store for 2026.
Smolt-to-Adult Rearing Trial: The Story so Far
Purpose of the Trial – Supporting Wild Spawning
Save the Spring’s smolt-to-adult supplementation trial is a pioneering, pilot intervention designed to support the recovery of wild spring-run dominant salmon populations which spawn in upper catchment areas, while avoiding the well-documented genetic and ecological risks associated with conventional hatcheries. The approach is precautionary and time-limited, aiming to preserve local genetic diversity and accelerate population recovery while longer-term habitat resilience work is underway – the main focus of Save the Spring.
The overwhelming weight of evidence shows that traditional stocking approaches, which breed and release juvenile salmon, carry significant risks, including; reduced fitness and a disruption of natural selection, lower marine survival rates, altered behaviour and long-term genetic impacts on wild populations – effectively risking suppressing natural wild salmon recovery. We therefore set out to trial a different approach to helping wild salmon, focused on enabling mate selection, spawning behaviour, egg hatching and juvenile life stages all to take place in the wild.
We will share findings with partners across the country and internationally to assist other river systems in their efforts to restore wild salmon populations.
Trial Recap – Where We’re At
In autumn 2025, 75 mature adult salmon originally collected as smolts from the River Muick in spring 2024, were successfully returned to their natal river to spawn in the wild. These fish were reared to maturity in saltwater tanks at the Marine Environmental Research Laboratory (MERL), part of the Institute of Aquaculture at the University of Stirling.
Prior to this, spring 2025 saw us collect a further 100 smolts (estimated to represent just 1.7% of the total Muick smolt run) which were carefully transferred to MERL, with plans for their return to the river as mature adults in autumn 2026 and 2027. Fish health and welfare have been prioritised throughout, with high survival rates, routine veterinary oversight, and comprehensive health screening prior to transport and release.
The project has been delivered through strong collaborative working with the University of Stirling, with open engagement and scrutiny from regulators, scientists and practitioners, ensuring that the work is transparent, well-governed and aligned with best practice. We also report to, and receive feedback regularly from, an international Scientific Advisory Group.
Wild Atlantic salmon smolts are safely captured and reared at MERL until they are mature adult salmon.
These adults are released back into the wild to spawn naturally.
What was the outcome for released fish?
Key Facts, Figures and Observations
Out of the 75 fish successfully released into the River Muick, comprising 43 females (57%) and 32 males (43%), 48 fish were fitted with radio transmitters to allow detailed monitoring of their post-release movements, habitat use and likely spawning locations, while providing valuable insight into behaviour following reintroduction.
Seven fish were released above the Linn of Muick falls, and the rest below – matching the locations of their initial capture. Four fixed radio tracking stations were set up at strategic points throughout the Muick to record fish movements, while a fifth unit was deployed as a portable device, allowing us to track the fish on foot. Radio tracking proved highly successful, with 47 of the 48 tagged fish subsequently detected. The single missing fish was likely either predated or experienced an early transmitter failure.
The released females had an average weight of 950g, with the largest fish at 1.5kg. This was smaller than we would have hoped for, but nevertheless, based on an estimated 1,500 eggs per kilogram, this represents a potential contribution of approximately 60,000 eggs to the river system. The largest of our male fish weighed 2.3kg.
The radio tracking programme has already revealed some fascinating behavioural insights.
Map showing the movement of fish over time. The red towers show the locations of the four fixed receiving stations, with clustered points around the two uppermost stations as they detected fish multiple times. Individual observation points are coloured according to date.
Evidence of Spawning
At least three of our fish moved into the Allt Cholzie, one of the primary spawning tributaries of the Muick. A dead female was found who was likely to have spawned with one of our male fish still present in the area. One of the three in this tributary had previously been at the Linn of Muick for several days, so moved back downstream to get to the Cholzie.
A female fish found in November in the Allt Cholzie after a rapid, multi-km movement. Few eggs remained in her indicating she likely spawned.
Despite eight fish from the lower release sites making it as far upriver as the Linn of Muick, only one fish successfully passed up through the fish ladder into the upper catchment – a large male that spent more than three weeks below the falls before finally ascending in November (the team even had to nickname him as we got to know him well!) Fish passage up the ladder was lower last year in general, probably linked to low flows.
Once upstream, he moved rapidly through the catchment and was later observed near important spawning gravels close to Loch Muick, where he was seen swimming alongside a wild fish of similar size.
Another male, initially released above the falls, stayed close to its release site for much of the season while occasionally making exploratory upstream movements. After weeks of uncertainty over whether the fish was still alive, it was finally observed in good condition and full of energy, showing the darker colouration typical of salmon adapting back to river life from the sea.
A wild fish (left) alongside a fish from our trial (right) in a known spawning location in the upper River Muick. The trial fish had successfully ascended the Linn of Muick fish ladder.
Genetic Sampling will be Key
Key to measuring the impact of the trial over the long term will be juvenile surveys and genetic monitoring.
Genetic samples were taken from every fish which has entered the trial. Through future juvenile survey work and genetic sampling, we will be able to determine the contribution to the population made by fish in the trial, matching juveniles to their parents.
After conducting the trial for several years, we will be able to understand the impact that our efforts made.
2026 Update – Recent Activity
It’s been a tense few weeks on Deeside as we waited for the weather to change and water temperatures to rise, prompting the smolts to begin their migration journey. All efforts were focused on bringing in the third cohort of fish for the smolt to adult supplementation trial – fish which will be released in 2027-2028.
Finally, on the last weekend of April, the migration began. We have now weighed, measured and PIT tagged 100 smolts, and they have safely arrived at MERL. The fish were lively and in excellent condition when they arrived at their destination and underwent gradual adaptation to seawater in their own dedicated accommodation tanks. We are excited to share their growth and development with you over the next 12–18 months.
Signs of Faster Growth in Cohort 2
After the first cohort of fish released were not quite as large as we had hoped, growth with cohort 2 (to be released in 2026-2027) is looking much more positive.
Overall, the signs are cautiously optimistic, with a good number of fish already reaching a healthy weight of around 2kg, equal to the largest salmon from cohort one that was returned to spawn in the wild last September. It will be interesting to see if this better growth translates into a difference of age at maturity, though it is also possible that not all fish will mature this year.
With around six months of feeding and growth still ahead, we hope to see continued progress in size, health and signs of maturity before the fish are returned to their birthplace on the River Muick in the autumn. The aim is to give them the best possible chance of spawning in the wild successfully.
We continue to learn valuable lessons with each cohort of fish, and continue to move ahead with cautious optimism. Thank you to everyone who has been enquiring about the fish returned last autumn, it’s fantastic to see such strong interest in the project! Despite financial challenges, our important trial continues to move forward.
